Ask for namespaces dir name when creating a new `multi` namespace.
Ask for namespaces dir name when creating a new `multi` namespace.

For new namespaces of type `multi` a new top level dir that contains
the actual namespace dirs is required. Previously the helper
always used the passed namespace name for this, i.e.:

    kubler -w /tmp/ new namespace foo

..would result in `/tmp/foo/foo/images/`, which is confusing.

edannenberg/gentoo-bb,edannenberg/kubler,edannenberg/gentoo-bb